Scientists are fairly confident that global warming increased the severity and likelihood of extreme heat events in 2013 in Australia, New Zealand, South Korea, Japan, China and Europe. Credit: Benjamin Jakabek via flickr

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Blistering heat waves recorded around the globe in 2013 were linked to human-caused global warming, according to a broad survey of studies on extreme weather events published yesterday.

But the studies could not link climate change as clearly to heavy rainfall, droughts and storms. For instance, the link between the three-year-long California drought and climate change remains to be deciphered by future research.

The studies are from a National Oceanic and Atmospheric Administration-led effort to explain the role of climate change in 16 extreme weather events in the United States and elsewhere. This is the third year for the "attribution" studies, which were published yesterday in the Bulletin of the American Meteorological Society.

Attribution research is relatively new, and scientists are still learning how to untangle the contribution of long-term global warming in a given weather event. The challenge lies in the fact that natural variability is always a part of any extreme weather event, so when scientists do attribution exercises, they are trying to discern the human signal out of the noise. The procedure is more straightforward for heat extremes than for complex events like storms and weird rainfall patterns.

"It is not ever a single factor that is responsible for the extremes that we see; in many cases, there are multiple factors," Tom Karl, director of NOAA's National Climatic Data Center, said in a press call.

'Rather striking' climate link to Australian heat waves
Because temperature extremes are easier to decipher, scientists are fairly confident that global warming increased the severity and likelihood of extreme heat events in 2013 in Australia, New Zealand, South Korea, Japan, China and Europe.

Attribution studies are meant to help policymakers understand whether an extreme weather event is likely to repeat in the future. If the studies suggest a particular type of event, such as blizzards, is linked to climate change, then policymakers can choose the relevant adaptation strategies, Karl said.

"If you are focusing on a blizzard in South Dakota in the spring, then the question is, should we prepare for more of those?" Karl asked rhetorically.

The clearest climate signals emerged in Australia, which is the driest continent on Earth. In 2013, the nation faced its hottest year on record since measurements began in 1910. The year was also exceptionally dry, leading to the colloquial name the Big Dry.

Five teams of scientists tried to explain the heat wave using different methodologies. All the studies found that global warming had increased the likelihood of record temperature extremes.

"I think the results from the Australian studies are rather striking," said Peter Stott, a co-editor of the report from the U.K. Met Office Hadley Centre. "[The studies] have said the chances of observing such extreme Australian temperatures in a world without anthropogenic global warming is almost impossible."

Still questions on Calif. drought
Many groups seeking action on climate change are eager to link that to the parched conditions now affecting California. But there is widespread disagreement among scientists on whether climate change is to blame (ClimateWire, Jan. 3).

"A clear picture of how exactly long-term climate change impacted the California drought is yet to emerge," Stephanie Herring of the National Climatic Data Center, the lead editor of the report, said during yesterday's call.

Three separate studies released yesterday examine the climatic drivers behind the drought.

All three point to the same atmospheric mechanism behind the lack of precipitation last winter. A persistent band of high pressure, sometimes called the "ridiculously resilient ridge," planted itself off California's coast. It effectively blocked rainstorms from the north Pacific Ocean that usually travel inland between October and May to water the state's crops and replenish its reservoirs.

But what caused this phenomenon -- and whether that cause is climate change -- remains unclear.

One study, led by Chris Funk of the U.S. Geological Survey and the University of California, Santa Barbara's Climate Hazard Group, looked at long-term warming of the sea surface in the North Pacific. Using climatic models, the researchers did not find a strong link between a steadily warming ocean and the atmospheric conditions leading to the lack of precipitation.

A second study, led by Hailan Wang of NASA's Goddard Space Flight Center, used different model simulations and came to a similar conclusion: While a warming sea surface did make it more likely that a high-pressure ridge could form, the signal was not strong enough to explain its extreme nature. Moreover, the models also found that warming leads to more humidity over the Pacific, which should have caused more rainfall, not less.

These two effects, the authors wrote, "appear to counteract each other, contributing no appreciable long-term change in the risk for dry climate extremes over California since the late 19th century."

Record-breaking heat likely makes dry conditions drier
The authors of the third study, however, did claim a strong link between climate change and the precipitation-blocking band of high pressure. Led by Stanford climate scientist Noah Diffenbaugh, the researchers used computer models and statistical modeling, concluding that without human-caused warming, the "ridiculously resilient ridge" would have been unlikely to form (Greenwire, Sept. 29).

But Marty Hoerling of NOAA's Earth System Research Laboratory questioned this result. Hoerling said the study did not prove that the high-pressure ridge was exceptional when compared with pressure increases taking place across the globe as the planet warms.

Hoerling, who co-edited the larger report, also noted there has been recent research finding that climate change will lead to an increase in rainfall over California.

"Different methods can give us different perspectives on what factors were at play, and that's what happened in these studies," Hoerling said.

Karl of the National Climatic Data Center said none of the studies addressed the contribution of heat waves to the drought. It is possible that the two could be linked, he said.

"Past work has shown that much warmer temperatures do tend to prolong the drying season and exacerbate drought," he said.

Storms also a question mark
The attribution studies also looked into storms and rainfall extremes, but the complexity of atmospheric processes during such events made it difficult for scientists to decipher the role of climate change.

For instance, the models did not tie the record Colorado floods after five days of heavy rainfall in September 2013 to climate change (ClimateWire, Oct. 30, 2013).

Hoerling was one of the scientists who analyzed the floods. He and his colleagues initially theorized that an increase in water vapor in the air due to global warming, as predicted by climate models, could have led to greater rainfall.

But when the scientists did the analysis using a particular climate model, they found that global warming has likely decreased the likelihood of heavy precipitation in September. And that was despite an increase of water vapor in the air by 6 percent, Hoerling said.

The case illustrates the challenges involved in attributing precipitation extremes and storms, which are caused by multiple factors, Hoerling said. Scientists need to examine such events using many different models to decipher the role of climate change, he said.

"Even the information published in this report should not be final word on this matter," he said.

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